The sand dollar is a part of the phylum Echinodermata. It is one of the main species that can be found in the order Clypeasteroida. Much like the sea urchin and other invertebrates in this class, it has a radial symmetrical body and has a water vascular system. The name "sand dollar" comes from its the shape of its body which takes on a large, coin-like appearance.
Sand dollars were created on the fifth day of creation, along with the other aquatic animals of the sea, as stated in Genesis 1:20-23.
When sand dollars are found in the sand on the beach, the velvety covering made up of tiny spines it once had is usually completely washed away and the color becomes somewhat "bleached" out by the sun. However, most people that come across sand dollars don't realize that these creatures were once alive at the bottom of the ocean. It is clear that the animal takes on a different appearance when it is alive on the sandy ocean floor than when it is found dead on the beach. 
Sand dollars can usually be found in a cluster at the bottom of the ocean where the sand is soft. They chose this particular area mainly for comfort. Here, many of the sand dollars will live and, eventually, reproduce with one another.
An old myth about sand dollars states that they were the washed up remains of a mermaids coin. 
Today, many people collect sand dollars that are found on the beach and, in some cases, these dead creatures can be found on display in stores. Perhaps the most well-known fact about sand dollars is the "doves" that can be found inside the sand dollar when it is broken.
The sand dollar's shell, also known as the test, has a radial symmetrical form that contains the shape of flower petals in the central most part of the top. The structure of its hard body is made completely out of "calcium carbonate plates". Much like the other animals in its class (Echinoid), the under side of the creature is where the mouth is located. This simply means that it eats from its stomach area rather than having to turn completely over onto its topside to eat. Protecting the mouth and the entire body are several tiny spines that aid the animal in capturing food.
Sand dollars also possess many tube-like feet which are used for gills. Though this animal seems sessile, it is not. Much like the sea urchin, the sand dollar actually does move. It does so by taking water into its body through 5 pores that take the shape of a flower on the top of its body. These pores help with the water vascular system and allow the sand dollar to be mobile.
When a dead sand dollar found on the beach is broken, white pieces that are in the shape of doves can be found inside. These "doves" were once the sand dollars teeth. 
Since sand dollars are generally small, they can only devour small things. Their main source of food consists of tiny floating particles that pass by them in the water. They latch onto these small particles with the black spines on the bottom half of their body that surround the mouth. Tiny cilia carefully direct the food particles into it's mouth where it is broken down into pieces which can be digested easily. This is why the sand dollar is usually found half buried in the sand with its mouth still uncovered; it makes for easy food capture. 
Sand dollars reproduce sexually, much like the sea urchin. When they are ready to reproduce, they release eggs or sperm, depending upon if it is a female or male, into the water through its pores. Here the two meet and become a single fertilized product and soon after, larvae is produced. The larvae that is made from the fertilized egg will float through the water until it comes to the next stage of its life cycle. Sand dollars will go through several stages before they even take on the form of a sand dollar and receive the familiar form of a test. 
When the sand dollar eats, it uses several of its black spines, which are connected to the underside of its body, to capture the food. From here, it passes the food on to the tiny hairs referred to as cilia. The cilia directs the food into its mouth where it is further passed on to be digested.
The small hairs located on the bottom of its test not only aid in the eating process, but they also allow it to move slowly through the sand or to bury itself to hide from its predators.
When a sand dollar is threatened, it will try to flee the scene to get away from danger. However, many times this does not work because its predators are often times faster than it is. Main hunters of sand dollars include starfish and snails.